The present invention reltates to a demodulator and, more particularly, to a demodulator for demodulating a quadrature-amplitude modulated signal.
A quadrature-amplitude modulation (QAM) system features a unique capability of transmitting a large amount of data per unit frequency of the occupied bandwidth of a carrier passband and, for this reason, it has been extensively used for large capacity radio communications in which the occupied frequency bandwidth is severely restricted. A prior art demodulator for demodulating a QAM signal includes an orthogonal detector which detects a four-phase modulated signal, which is one of QAM signals, to produce two demodulated signals, and two analog-to-digital (AD) converters which discriminate respectively the two demodulated signals responsive to a clock to produce two different data signals each. The data signals from one of the AD converters and the clock are applied to a control circuit which then generates a control signal. A voltage controlled oscillator (VCO) oscillates the clock having a frequency which is controlled by the control signal. Of the two data signals outputted by each of the AD converters, at least one serves as an error signal representative of a deviation of the value of the associated demodulated signal at a sampling point of the AD converter from a normal value (normal level).
In a demodulator designed for use with QAM signals, the influence of insignificant deviations of the sampling point may be compensated for by use of an adaptive transversal equalizer of the type described in, for example, Papers dedicated to The 1984 National Meeting of The Communications Section of The Institute of Electronics and Communication Engineers of Japan, Fascicle 3 (1984-10), pp. 3-23 and 24. An adaptive transversal equalizer is capable of compensating for distortions due to delay and attenuation in a transmission path by automatic control and in conformity to particular conditions of the transmission path. The tap constant of such a special equalizer is determined by use of the previously mentioned error signal.
Where the prior art demodulator is used in combination with an adaptive transversal equalizer, the above-stated error signal bifunctions as an error signal adapted to control a clock synchronizing circuit of the demodulator and an error signal adapted to control the equalizer. In such a condition, a control loop associated with the demodulator and a control loop associated with the equalizer affect each other to become astatic, the equalizer significantly deteriorating the demodulation eye pattern. Stated another way, should the prior art demodulator be used together with an adaptive transversal equalizer, the control over the demodulator and that over the equalizer might each be prevented from converging to the optimum point.